WASHINGTON—Massachusetts Senator William “Mo” Cowan joined the Congressional Black Caucus Monday, becoming the group’s only member in the Senate.

Cowan, appointed by Bay State Governor Deval Patrick in January to fill John Kerry’s vacated Senate seat, joins 42 other members of the caucus. The reliably liberal voting bloc was formed in 1971 to advocate policies beneficial to African-Americans.

“I have long admired the work of the [Congressional Black Caucus], especially their efforts in education, equal justice, and addressing health disparities,” Cowan said in a statement. “I’m looking forward to joining the other members in the continuing our work to ensure that America remains the land of opportunity for all.”

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Cowan’s appointment marked the first time in Senate history that two African-Americans are serving at the same time. Republican Senator Tim Scott of South Carolina—also an interim replacement—chose against joining the caucus upon election to the House in 2010, saying he’d rather focus on issues that affect all Americans.

The Tea Party favorite downplayed race during his congressional campaign, which included a primary battle against Paul Thurmond, son of the longtime US senator and segregationist Strom Thurmond.